Receiving a green card doesn't mean a person is completely done with immigration law issues regarding U.S. permanent residency. For one, criminal issues or long absences from the country can threaten one's green card status. Another reason is that a green card doesn't last forever. Rather, green cards have expiration dates.
What this expiration date is depends on the type of permanent resident a person is. For a regular permanent resident, a green card lasts 10 years.
The expiration date is different for individuals that are conditional permanent residents. Green cards given in relation to conditional permanent residency only last two years. This typically happens when a green card is acquired through marrriage and that marriage is less than two years old.
Now, for regular permanent residents, there is a process they can go through as their green card is nearing its expiration date to renew their green card for a fresh new 10-year period.
Conditional permanent resident green cards, on the other hand, are ineligible for renewal. However, there is a process in which a conditional permanent resident can request to have the conditions from their residency removed. If such a request is granted, the person becomes a regular permanent resident, and thus the green card expiration date and renewal rules that apply to them shift to those for regular permanent residents.
When a person's green card is nearing its expiration date, it is important for them to be aware of what options they have, given their own personal immigration circumstances, for maintaining their permanent resident status and what processes and rules are connected to such options. Immigration lawyers can provide guidance to permanent residents on issues such as green card renewals and condition removal requests.
Sources: U.S. Citizenship and Immigration Services, "After a Green Card is Granted," Accessed Nov. 3, 2015
U.S. Citizenship and Immigration Services, "Conditional Permanent Residence," Accessed Nov. 3, 2015